This invention relates to ultrasonic diagnostic imaging system probes, and in particular to an ultrasound probe having an integrated acoustic standoff that is suited for imaging and diagnosing organs of the body and arterial vessels during surgery.
Ultrasonic diagnostic imaging systems are in widespread use for performing ultrasonic imaging and measurements of the human body through the use of ultrasound probes. Ultrasound probes are generally used external to the body in non-invasive procedures but can also be used internal to the body being examined during surgical procedures. Ultrasound probes are used to view the internal structure of a body by creating a scan plane or volume, which is produced from an array of transducers. The transducers transmit pulses of acoustic energy or beams into the body and receive returning echoes of that energy as they are reflected from internal structures of the body. A correctly focused beam is achieved by using an acoustic lens. The focal zone is the range over which the beam is sufficiently narrow relative to the anatomical features in the area interest. Beyond the focal zone the beams begin to diverge and may become larger than the relevant anatomical features. These relative beam sizes determine the resolution of the ultrasound image.
In surgical procedures, such as vascular surgery, the ultrasound probe may be used to image and diagnose the interior of arteries, or the blood flow of a vessel or organ. In other surgical procedures, such as transplants, for example, the ultrasound probe may be used to verify successful attachment of renal arteries. Similarly, in non-invasive procedures such as musculoskeletal or peripheral vascular procedures, the ultrasound probe may be used to image or diagnose an area located near the surface of the skin. Such procedures require the ultrasound probe to have focal properties that enable it to produce a focused image of tissues and structures that are in immediate contact with the probe. Such focal properties include having a focal zone that is in immediate contact with the probe.
Lens and standoff materials have been used alone or in combination to attempt to provide a probe with such a focal zone. Lenses are used because the focal zone can be controlled by placing a lens in the path of the ultrasonic beam to cause the beam to focus or converge at a faster rate as the beam travels away from the transducer. If the beam passes through a convex lens having an acoustic velocity slower than the medium that it is being transmitted into, the beam will converge even faster and will place the focal zone at a shorter distance from the transducer. Standoffs are used between the transducer and the surface being examined to place the focal zone at a certain distance from the transducer. An optimal standoff or standoff-lens combination for the procedures described above would place the focal zone immediately below the area in immediate contact with the probe. As discussed below, there are many factors such as standoff material, standoff height and standoff-lens arrangements that affect the location of the focal zone.
An ultrasound probe that attempts to place the focal zone at the area in immediate contact with the probe is described in U.S. Pat. No. 5,381,795, the contents of which are incorporated herein by reference. That patent describes an ultrasound probe with an RTV rubber boot that encapsulates the transducer and also forms an external standoff and lens for providing a focal zone in the area near the surface contacting the probe. While the boot provides a standoff that can be easily cleaned, disinfected or sterilized, the arrangement of the standoff and lens in that patent does not provide an ultrasound probe that optimally places the focal zone at or immediately below the surface contacting the area of interest.
As the ultrasonic beams leave the transducer of that patent, the beams may slightly converge as they pass through the flat standoff. Since the convex lens is located at the patient contact surface of the probe the focusing effect of the lens itself does not begin until somewhere past the contact surface. Such an arrangement of the standoff and lens does not optimally locate the focal zone for imaging areas that are in immediate contact with the probe.
In addition to the arrangement of the standoff and lens, the choice of standoff material can also affect the location of the focal zone. In choosing a standoff material for a probe with a focal zone in the area in immediate contact with the probe, it is desirable to have a standoff with optimal acoustic characteristics (impedance, velocity and attenuation). In a standoff with optimal acoustic characteristics the impedance and velocity will closely match the characteristics of body tissue and the acoustic attenuation of the standoff will be minimized as much as possible. In reference to acoustic characteristics, an optimal and desired standoff material will have a longitudinal velocity between 1.4 to 1.6 mm/psec, an impedance between 1.4 MRayls and 1.60 MRayls, and an attenuation no greater than 0.10 dB/mm/MHz.
When the attenuation is minimized and the impedance and velocity characteristics of a standoff closely match body tissue, the quality of the ultrasound image is higher because the ultrasonic beams and returning echoes are not distorted, or reflected and are minimally attenuated as they pass through the standoff. Also, with optimal acoustic characteristics, the ultrasonic beams traveling through the standoff will be able to converge at a faster rate to form a focal zone located closer to the area in immediate contact with the probe, if desired.
In probes that require the standoff to come into contact with the surface being examined, the biocompatibility, and chemical and mechanical properties of the standoff material become more important. In such instances, the standoff material should be biocompatible, have strong chemical resistance and a strong mechanical or abrasive resistance, which makes the standoff easier to manipulate, increases the durability and usable life of the standoff, provides a material that can be easily cleaned, disinfected or sterilized, and provides a material that can act as an electrical insulator.
A problem arises when choosing a standoff material with strong chemical and mechanical properties because as such properties are optimized, the acoustic characteristics become less than optimal. For instance, a standoff material comprised of RTV rubber will have chemical and mechanical properties that make the standoff easier to manipulate, clean, disinfect or sterilized but such material will have less than optimal acoustic characteristics. That problem creates a conflict in standoff material choice where a focal zone is desired in the area in immediate contact with the probe and where the standoff material must have strong chemical and mechanical properties. In such cases a standoff with optimal acoustic properties is desired to increase image quality as explained above and to provide greater flexibility in choosing the thickness of the standoff for focal zone placement. As the chemical and mechanical properties are made stronger, the image quality is negatively affected and the maximum thickness of the standoff is limited, thus decreasing flexibility in placing the focal zone in the area in immediate contact with the probe.
Also, standoffs that come into contact with the surface being examined are generally disposable and must be replaced and stocked, and may require additional manipulation by the user of the probe, which distracts the user from the procedure at hand. It is important in surgical procedures that the size and shape of the probe permit the user of the probe to manipulate the probe with ease.
Accordingly, it is desirable to have an ultrasonic probe with a standoff integrated into the probe and strategically arranged with the lens for imaging and diagnosing areas of interest that are in immediate contact with the probe. The standoff materials of such a probe should have optimal acoustic properties with little or no regard for the chemical or mechanical properties such that the standoff may be thicker, if desired, to place the focal zone at the area in immediate contact with the probe. It is also desirable that the probe will provide reliable protection against electrical shock, may be properly sterilized and disinfected, and that the standoff can be used for the life of the probe. It is also desirable that the probe is easy to manipulate.
In accordance with the principles of the present invention, an ultrasound probe is provided for imaging and diagnosing areas of interest that are in immediate contact with the probe. The probe provides an integrated standoff comprised of a rubber material having optimal acoustic characteristics. The lens is directly applied to the transducer and the standoff is applied to the lens such that the focal zone is placed at the area immediately below the patient contact surface of the probe. The lens material also encapsulates the transducer and provides reliable protection against electrical shock. The standoff is also separated from the surface being examined by a cap comprised of a biocompatible elastomer having high chemical and abrasive resistance that enables the probe to be easily sterilized and disinfected and provides further protection against electrical shock. The use of a standoff with optimal acoustic properties in combination with the arrangement of the lens, standoff and cap provides a probe with a focal zone placed at the area immediately below the probe. The lens, standoff and cap are spatially arranged to minimize the volume of the distal transducer section and overall probe such that the probe is easy to manipulate.